Air quality is an ongoing matter of concern, particularly in metropolitan and industrialized areas. Problems of air pollution are often associated with levels of carbon dioxide (CO2) and particulate matter; however, nitrogen oxides (NOx) are also significant contributors to air pollution.
Nitrogen dioxide (NO2) is a reactive gas that is mainly formed by oxidation of nitrogen monoxide (NO), and high temperature combustion processes (such as in car engines and power plants) are major sources of NOx (i.e., the sum of NO and NO2). While the majority of NOx emissions are in the form of NO, a significant amount is in the form of NO2. It is known that negative health effects can be linked to short-term exposure to NO2 (e.g., changes in lung function in sensitive population groups) and long-term exposure (e.g., increased susceptibility to respiratory infection). Excess deposition of atmospheric nitrogen can lead to a surplus of N nutrient in ecosystems, which can lead to eutrophication in terrestrial and aquatic ecosystems. Nitrogen oxides also play a major role in the formation of ozone, and by contributing to the formation of secondary inorganic aerosols (i.e., through nitrate formation), NOx also can contribute the particulate matter concentration in the air.
Various steps have been taken to reduce NOx pollution, including controlling allowable NOx emissions. One NOx abatement technology that has been introduced relates to photocatalysis of NOx utilizing a catalyst in the presence of light, water, and oxygen. For example, when titanium dioxide (TiO2) is exposed to ultraviolet (UV) light, electron-hole pairs are generated, facilitating reduction and oxidations reactions through the formation of adsorbed free radicals on the TiO2 surface. The highly reactive radical species can degrade pollutants (such as nitrogen oxides, sulfur oxides, and volatile organic compounds) into relatively harmless substances (e.g., formation of nitrates through oxidation of NOx) without substantial consumption of the TiO2 catalyst.
Photocatalytic paints and similar coating materials incorporating a photocatalyst (such as TiO2) have been developed as potential media for NOx abatement and have shown varying levels of success. Nevertheless, there remains a need for further technologies for increasing NOx reduction in the air.